Intravital microscopy will be used to study the terminal vascular bed in the bulbar conjunctiva of the eye and in the nail fold of the fingers and toes in two sets of patients: a) Randomly selected group of individuals who are part of an epidemiological study of the natural history of a middle-aged sector of the population; (b) Selected individuals with specific peripheral vascular problems - e.g., diabetes mellitus, arterial occlusive disease, hypertension. Recently developed methods for quantitative morphometry, volumetric flow rate and vessel diameter will be used to obtain quantitative indices of the status of the microcirculation. Vascular data will be recorded on video tape and then subjected to analysis. Models will be developed to permit the formulation of distribution functions of important regulatory features. It is proposed also to develop a non-invasive procedure for estimating microvascular pressures. Blood rheological data will be obtained on in-vitro test systems and compared with in-vivo flow data to define the contribution of changes in the flow properties of blood during disease.